This invention relates to an improved apparatus for making diamond-coated tools.
Diamond-coating of tools has been performed for a number of years. Diamond-coating provides a strong and highly abrasive tool useful for cutting or otherwise abrading surfaces.
One method of making a diamond-coated tool utilizes chemical-vapor deposition of carbon onto a substrate under conditions that are sufficient to grow diamond on the surface of the substrate. Typically, substrates to be diamond-coated are placed in a batch reactor in close proximity to several resistance heating filaments, and the reactor is slowly heated to the appropriate temperature. A mixture of hydrogen and hydrocarbon gas such as methane is heated to at least the temperature at which hydrogen gas dissociates and forms elemental hydrogen. Elemental hydrogen reacts with the hydrocarbon to form elemental carbon, and when this hot gaseous mixture is circulated around the substrate, carbon is deposited on the substrate.
Several factors are important to successfully deposit carbon and grow diamond on the substrate's surface. The gases immediately surrounding the substrate must be hot enough to dissociate hydrogen and subsequently assist formation of elemental carbon. The substrate must also be sufficiently hot for a period which is long enough to affix elemental carbon to the substrate's surface or to other carbon deposited on the tool and to crystallize the carbon into diamond. The temperature at which the reaction occurs must be closely controlled to optimize diamond growth. The integrity of the resistance heating filaments is thus critical in achieving and maintaining the correct temperature for the duration of the reaction. If a heating filament breaks or sags, the temperature in the vicinity of the heating element drops, thereby creating unanticipated thermal gradients in the reactor and introducing substandard conditions for carbon deposition and diamond growth.
The condition and position of heating elements is therefore a critical factor in diamond growth. Resistance heating filaments are delicate and are easily broken during installation, heat-up, and/or normal operation of the reactor. Typical resistance heating filaments are approximately 0.002-0.06 cm thick and 30 cm long. Filaments expand when heated from room temperature to their typical operating temperature of 2500.degree. C. and are thus subject to thermally-induced stresses that can break the filaments. Also, during carbon deposition, resistance heating filaments can react with carbon-bearing gases contacting the filaments and form a carbide, further lengthening and embrittling the filaments. It has been reported in the literature that typical tungsten resistance heating filaments expand about 20% in length because of carburization. Filaments can sag or droop and short-out because of contact with a ground source such as the reactor floor or structure. Further, filaments that are not taut are more likely to break because of vibrations that are naturally present in the reactor. Thus, heating filament expansion and breakage is a major source of inconsistent carbon deposition and diamond growth, and filament breakage is a major contributor to substandard diamond-coated cutting and abrading tools. Carburization of heating filaments usually requires new heating filaments to be installed after a batch of tools is made.
There have been a number of attempts to control filament breakage. U.S. Pat. No. 4,953,499 states that vertical, pre-stressed, curved filaments alleviate filament breakage or filament/filament or filament/substrate contact and thereby compensate for thermal expansion and expansion caused by filament carburization. The filaments are typically prestressed in a suitable jig. However, this method is expected to result in significant breakage of brittle filaments such as tungsten filaments during the pre-stressing step, thereby adding to the cost of producing diamond-coated tools. Also, the bends introduced into the filaments would be expected to be mechanically weak, resulting in more filament breakage in use in the reactor than straight filaments would provide. It is also anticipated that these bends create internal stresses in the filaments that, upon heating, cause the filaments to bend in unexpected and unwanted directions, thereby misaligning the filaments. Further, these curved filaments are mounted to non-adjustable fixed electrodes on each end, causing the filaments to bend even more as they gain mass and length due to carburization.
A further attempt to reduce the incidence of filament breakage is disclosed in U.S. Pat. No. 4,958,592. '592 states that a vertical heating-element may be maintained without coiling and without flexing when connected to a counterbalancing weight assembly.
The '592 counterbalancing weight assembly is a complex assembly that is unadjustable during use and that consists of numerous elements. A filament tensioning rod runs through a low-friction bearing said to have axial and longitudinal freedom of movement. The tensioning rod is connected to a single filament on one end and to a lever arm on the other end. The lever arm has a counterweight on the end opposite to the end to which the tensioning rod is attached, and the lever arm is suspended about a fulcrum point by a cable. The mass of the lever-arm counterweight is determined by the mass of the tensioning rod and the distances of counterweight and tensioning rod from the fulcrum point. The cable suspending the assembly rides on a pulley and has a second counterweight attached to the cable that equals the sum of the tensioning rod and counterweight masses. The pulley suspends the whole assembly of cable, counterweights, lever arm, and tensioning rod in mid-air.
This counterbalancing weight assembly is expected to have numerous problems. First, the entire assembly is suspended in mid-air and is thus prone to rocking, swaying, and/or pivoting in any environment but one in which the air around the assembly is absolutely quiescent. A completely quiescent environment is quite difficult to achieve, especially since the '592 assembly is connected to a heated oven that creates natural convection currents in the air around the outside of the oven and around the counterbalancing weight assembly. Vibration from associated equipment will also invariably be transmitted into the counterweight assembly through the pulley, the reactor, or the air around the counterweight assembly. The '592 assembly is analogous to a dumbbell suspended from a cable, which cable is slung over a pulley and to which cable a mass equal to the dumbbell's mass is suspended. Any external forces that are transmitted to the suspended dumbbell set it into motion that is difficult to stop because of the large moment of inertia of a dumbbell. The rocking dumbbell is essentially free to pivot about the fulcrum in any manner. The dumbbell can rock up and down about the fulcrum; it can swing in an arc from side to side at the end of the cable; it can pivot about the cable; or it can make any combination of these movements. Any of these movements would be expected to induce significant forces (compression or tension) into the delicate filament attached to the end of the tensioning rod, subjecting the filament to stresses that could easily cause failure of the filament during reactor operation.
The '592 counterweight assembly also has no provisions to overcome hysteresis inherent in its design. The low-friction bearing disclosed in the '592 assembly has two-dimensional freedom of movement, so it can accommodate radial movements and longitudinal movements of the tensioning rod. However, the bearing cannot accommodate a three-dimensional motion such as a rocking motion of the assembly. The tensioning rod would likely bind in the bearing, particularly at the high temperatures to which the bearing is subjected at the reactor, until the lever arm moved to a position where the tensioning rod was no longer binding in the bearing. Then, the force would suddenly be instantaneously relieved axially and/or longitudinally. This could subject the filament to an unwanted force of magnitude sufficient to break the filament.
An additional problem of the '592 assembly is the precision required in setting-up the assembly. The mass of each counterweight must be precisely controlled, and the pivot point must also be precisely positioned to establish a balanced system and to prevent the assembly from subjecting the filament to a substantial and incorrect force during reactor operation.
Also, each '592 counterweight assembly is attached to only one filament. Typical reactors utilize at least eight to ten filaments and can have approximately 30 or more filaments, which are all replaced after a batch of diamond-coated tools are produced. Thus, many counterweight assemblies must be precisely fabricated or overhauled for each run, and after each run, the assemblies must again be precisely reconstructed and adjusted. The '592 counterweight assembly would consequently require substantial maintenance and adjustment between batches.
The '592 counterweight assembly is subject to many problems that would limit the system's ability to prevent premature breakage of filaments. The '592 patent states that "an appropriate system of spring biasing means may be employed to reduce the tension on [the] heater filament . . . to 0 at its incandescent temperature[,]" but '592 is silent on what the "appropriate system of spring biasing means" might be.
Another attempt to prevent filament breakage during operation is disclosed in U.S. Pat. No. 4,970,986. This system utilize a plurality of reactor-enclosed spring means attached to an equal plurality of movable electrodes to maintain an equal number of filaments taut. This system has numerous shortcomings.
Each spring is attached at one end to a fixed frame contained within the reactor and at its other end to its own movable electrode, also contained within the reactor. Each electrode is attached to a single filament. Each spring is presumably pre-tensioned prior to sealing the reactor and operating it. There are no provisions to allow adjusting the tension on filaments after servicing the reactor and closing it. There especially is no provision to adjust each steel spring as it carburizes and hardens in the hot carbon-bearing gases present in the reactor. The '986 patent recognizes that over-tensioning the spring can occur and provides a plug attached to each delicate filament in an attempt to prevent over-tensioning of the filament by the spring. However, if this plug is not precisely adjusted, the filament will be over-tensioned or under-tensioned at some point in its operating cycle, and if the set-screw for the plug is over-tightened, the filament is weakened.
Since there are multiple filaments, there are many opportunities to incorrectly adjust a plug and cause premature filament breakage. Each time that the filaments and/or springs are changed, the plug positions must be adjusted. Any variations in spring length, spring constant, or filament length used from one batch of tools to the next requires compensation for and adjustment of each new spring and filament. Further, the multiple spring, electrode, and guide bearing assemblies of the '986 patent require more time to be spent maintaining the reactor between cycles and provide more opportunities for equipment failure during operation.
The electrode and spring design in the '986 reactor also can create substantial vibrations in the filaments during reactor operation. Current passing through the filaments creates strong electric and magnetic fields around each filament, and the fields of adjacent filaments interact to repel the filaments from each other. The small electrode and flexible spring on the end of each filament allow fairly free lateral movement of that end of the filament. Thus, the free ends of the filaments can be whipped about by the electric and magnetic fields induced by flowing current, causing the filaments to vibrate strongly if the spring on each filament is only designed to absorb the growth of the filament. Thus, strong vibrations and consequent filament failure must be accepted, or springs must be selected to exert a much stronger force on the filaments to restrict the amount of vibration as well as to compensate for filament growth. Even with a stronger spring, the filaments in the '986 reactor experience vibrational forces in all directions, especially at locations near the ends of the filaments, creating stresses in the filaments that can break them. Because of stronger springs, the filaments are also subjected to more tension in the '986 reactor than would be present in a reactor designed to minimize such vibrations and which has to compensate primarily for filament growth. This additional tension can also contribute to failure of the hair-like heating filaments.
The '986 reactor has another short-coming. The copper power-supply braid attached to each movable electrode must be a fairly substantial braid in order to supply large quantities of power and also to withstand the high temperatures present near the 2000-2500.degree. C. filaments. The braid adds weight that the spring must overcome, and differences in braid weight must be taken into account when adjusting each filament/spring assembly between batches. Further, the braid can bind, especially at elevated temperatures, causing unpredictable stresses that the spring must try to overcome to maintain its filament taut.
The filament and reactor designs discussed previously have not provided designs that both adequately address premature filament breakage and provide a system that can be easily maintained and quickly fitted with new filaments between batches. In the '499 patent, the bowed filaments bow out even further and in unpredictable directions because carburization lengthens the filaments, creating more force on the filaments at the bend points (compressive force on the inside sections of the bends and tension on the outside sections of the bends) and causing misalignment of the filaments. In the '592 patent, the multiple counterweight balances are a maintenance-intensive system for tensioning filaments, and uncontrolled forces are easily introduced by normal operating conditions such as vibration and high temperature present at the reactor. In the '986 patent, many springs, electrodes, filaments, power-supply braids, and over-tension plugs must be changed and/or adjusted between batches, the filaments fatigue from substantial forces in varying directions because of vibration and/or strong spring tension, and carburization changes the spring constant of each spring during reactor operation. The prior art has not provided an easily-maintained reactor that effectively solves the problem of carburization of filaments and other components present in the reactor.
Therefore, one object of this invention is to provide an apparatus that can maintain a substantially constant force on the filaments in an array during operation of the filaments. Another object of this invention is to provide a simple, adjustable apparatus for maintaining a substantially constant force on filaments in an array over the life of the filaments, which apparatus requires little maintenance and equipment set-up between cycles. A further object of this invention is to provide a force on filaments in an array that is parallel to the axes of the filaments. These and other objects are apparent from the disclosure herein.